1. Field of the Invention
The present invention relates to a degradable adhesive film and a degradable resin composition. More particularly, the invention relates to a degradable resin composition used for the substrate of a polylactic acid based degradable adhesive film for surface protection which is used by being tentatively adhered on the surface of adherends including metal plates such as stainless steel and aluminum or their workpieces, resin coated woodboards, decorative laminates, wood and metal furniture and automotive bodies, or by being provisionally adhered on the surface of a semiconductor wafer in the fabrication step; a polylactic acid based degradable adhesive film for display which is printed or coated on the surface; a pressure-sensitive adhesive tape for vegetable tying materials and stationery products; and other degradable adhesive films for various uses.
2. Related Art of the Invention
Conventionally, adhesive films or tapes using films of polyvinyl chloride, polyolefin or an ethylene/vinyl acetate copolymer as substrate and mounting an adhesive layer on the substrate surface have been widely used as an adhesive film for surface protection which is used by being tentatively adhered on the surface of adherends including metal plates such as stainless steel and aluminum or their workpieces, resin coated woodboards, decorative laminates, wood and metal furniture, measuring instruments such as watches, and automotive bodies; as an adhesive film for semiconductor wafer fabrication which is used for protection by adhering on the wafer surface having a built-in semiconductor IC in the step of grinding or dicing the other surface (the back) of said wafer; and as an adhesive film for display which is printed or coated on the film surface.
For example, Japanese Laid-Open Patent HEI 2-107684 discloses a surface-protective film having an adhesive layer on one surface of a flexible vinyl chloride resin film comprising a high polymer plasticizer having a molecular weight of 500 or more. Japanese Laid-Open Patent SHO 61-10242 (U.S. Pat. Nos. 4,853,286 and 4,928,438) discloses an adhesive film for wafer fabrication which has an adhesive layer on the surface of substrate film such as an ethylene/vinyl acetate copolymer film or polybutadiene film which has a Shore-D hardness of 40 or less.
Japanese Patent Laid-Open Publication HEI 2-300281 discloses an adhesive film for protection which is tentatively adhered on the surface of an automotive body in order to prevent the surface from corrosion and deterioration by rain, dust or sea breezes during transportation and storage in the course of delivering new automotives (four- and two- wheeled painting-completed vehicles) to the customers. The adhesive film for automotive-protection is prepared by applying an adhesive to one surface of a film of general purpose resin such as polyethylene, polypropylene, polyvinyl chloride and an ethylene/vinyl acetate copolymer.
Conventionally, in the case of conducting a notice, advertisement or propagation by illustrating a letter, drawing, stamp or mark on a signboard and wall or decorating a car body or building, it is widely performed to stick on the above adherends said general purpose resin film which is printed, transcripted or painted with the letter etc.
As a result of the object for use, the above various adhesive films for surface protection are almost stripped and abandoned after protecting the surface for a prescribed period by sticking to the adherends. However, the above general purpose resin used for the substrate of these conventional adhesive films does not degrade or has a very slow degradation rate under the natural environment. Consequently, these films semipermanently remain when disposed after use by burying in soil, and impair the scenery or destruct the living environment of marine organisms when abandoned in the ocean. Further, these films require a large amount of energy for incineration and polyvinyl chloride films in particular have problems of developing toxic gas. As a result, waste disposal of these general purpose resin films has been a serious social problem.
A lactic acid base polymer has been already known as a hydrolyzable polymer or a bioabsorbable polymer. For example, Japanese Patent Publication SHO 49-36597 discloses an auxiliary surgical tool obtained from a lactic acid/glycolic acid copolymer consisting of 65.about.85% by weight of lactic acid units and 35.about.15% by weight of glycolic acid units. Japanese Patent Laid-Open Publication 62-501611 discloses a medical transplanting material obtained from a copolymer of caprolactone and lactide, a cyclic dimer of lactic acid.
Polylactic acid or a lactic acid/hydroxycarboxylic acid copolymer can be effectively hydrolyzed by atmospheric moisture and thus has recently been intended to apply to a fundamental component of degradable general purpose materials for disposable uses other than medical tools. Various information has already been obtained on the hydrolysis rate of polylactic acid and a lactic acid/hydroxycarboxylic acid copolymer. It is hence relatively easy to design materials so as to meet the degradation period required for disposable uses. For example, high molecular weight poly(L-lactic acid) is suited for a use period of about 6 months, and a lactic acid/glycolic acid copolymer is suited for uses where the use period is several days and quick decomposition is desired after use.
According to the knowledge of the inventors, however, outdoor use of conventional materials prepared from lactic acid base polymers such as polylactic acid and a lactic acid/hydroxycarboxylic acid copolymer generally leads to distinctly faster reduction of strength as compared with indoor use or uses in a dark place or living body. It has been found that phenomena such as embrittlement, rupture and disappearance occur earlier than expected. For example a film which was expected to hold its strength for at least 6 months at ambient temperature in view of information on the rate of hydrolysis was embrittled after outdoor use for about a month and could not perform its function any more. Further, the rate of acceleration of the decomposition could not be anticipated at all and the decomposition period was diversified.
As described above, earlier decomposition than prescribed degradation period sometimes results in a serious problem for utilizing a lactic acid based polymer such as polylactic acid and a lactic acid/hydroxycarboxylic acid copolymer as a degradable material. Such problem is a great disadvantage which cannot be overlooked at all.